Disconnectable driveline for all-wheel drive vehicle

ABSTRACT

A disconnectable driveline arrangement for an all-wheel drive vehicle includes a power take-off unit having a disconnect mechanism, and a drive module having a torque transfer device and a limited slip clutch assembly.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/919,439 filed Jun. 17, 2013 (now U.S. Pat. No. 8,597,150), which is acontinuation of U.S. patent application Ser. No. 13/471,560 filed May15, 2012 entitled “Disconnectable Driveline For All-Wheel Drive Vehicle”(now U.S. Pat. No. 8,469,854). The disclosures of the above-referencedpatent applications are incorporated by reference as if fully set forthin their entirety herein.

FIELD

The present disclosure relates generally to all-wheel drive vehicles andmore particularly to disconnectable drivelines for all-wheel drivevehicles.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

Many modern automotive vehicles, such as crossover vehicles, areavailable with an all-wheel drive (AWD) drivetrain that is based on afront-wheel drive (FWD) architecture. This optional drivetrainarrangement permits drive torque to be selectively and/or automaticallytransferred from the powertrain to both the primary (i.e., front)driveline and the secondary (i.e., rear) driveline to provide bettertraction when the vehicle is operated in inclement weather and onoff-highway road conditions. Such AWD vehicles necessarily are equippedwith a much more complex drivetrain which, in addition to the primarydriveline, must include the additional components associated with thesecondary driveline such as a power take-off unit and a propshaft.

In an effort to minimize driveline losses (i.e., viscous drag, friction,inertia and oil churning) associated with secondary driveline beingback-driven when no drive torque is transmitted thereto, it is known toincorporate a disconnect system that is configured to uncouplecomponents of the secondary driveline such as, for example, the rearwheels or the rear differential from the remainder of the secondarydriveline. To this end, there remains a need in the art for developmentof improved disconnectable drivelines for use in AWD vehicles.

SUMMARY

It is an aspect of the present teachings to provide a disconnectablesecondary driveline arrangement for use with all-wheel drive vehiclesthat includes a power take-off unit having a disconnect mechanism, arear drive module having a torque transfer device capable of providingdisconnect and torque biasing functions, a limited slip clutch assemblycapable of limiting speed differentiation between the secondary wheels,and a control system for controlling actuation of the disconnectmechanism, the torque transfer device and the limited slip clutchassembly.

In accordance with this and other aspects of the present teachings, anall-wheel drive vehicle can include a powertrain, a primary driveline, apower switching mechanism, a secondary driveline, and a control system.The powertrain can include a prime mover and a transmission having anoutput. The primary driveline is driven by the transmission output andis operable to direct rotary power from the prime mover to a pair ofprimary vehicle wheels. The power switching mechanism is operable underthe control of the control system in one of a disconnected mode and aconnected mode. The power switching mechanism is operable in itsconnected mode to direct rotary power from the transmission output tothe secondary driveline. The secondary driveline can include a reardrive module and a propshaft that couples an output of the powerswitching mechanism to an input of the rear drive module. The rear drivemodule can include a secondary differential interconnecting a pair ofaxleshafts to a pair of secondary vehicle wheels, a torque transferdevice operably disposed between the input and the secondarydifferential, and a limited slip clutch assembly operably disposedbetween the secondary differential and one of the axleshafts. The torquetransfer device is operable under the control of the control system inone of a disconnected mode and a connected mode. The torque transferdevice is operable in its connected mode to direct rotary powertransmitted by the power switching mechanism to the secondarydifferential. The limited slip clutch assembly is operable under thecontrol of the control system in one of an open mode and a locked mode.The limited slip clutch assembly is operable in its locked mode toinhibit relative rotation between the axleshafts. When the powerswitching mechanism and the torque transfer device are in theirdisconnected modes, rotary power is only transmitted to the primaryvehicle wheels. The torque transfer device is operable in itsdisconnected mode to prevent the secondary vehicle wheels and thesecondary differential from back-driving the input of the rear drivemodule, the propshaft, and the output of the power switching mechanism.The power switching mechanism is operable in its disconnected mode toprevent the transmission output from driving the output of the powerswitching mechanism and the propshaft.

In another form, the present teachings provide a drivetrain for anall-wheel drive motor vehicle. The drivetrain can include a firstdriveline, a power switching mechanism and a second driveline. The firstdriveline is configured to drive a pair of first vehicle wheels andincludes a first differential and a pair of first axleshafts. The firstdifferential has a first differential case and a pair of first outputgears that are driven by the first differential case. The firstaxleshafts are drivingly coupled to the first output gears and to thefirst vehicle wheels. The power switching mechanism has an input shaftthat is configured to rotate with the first differential case, an outputpinion shaft, and a disconnect mechanism. The disconnect mechanism isoperable in a disconnected mode, which inhibits transmission of rotarypower between the input shaft and the output pinion shaft, and in aconnected mode that permits transmission of rotary power between theinput shaft and the output pinion shaft. The second driveline isconfigured to drive a pair of second vehicle wheels and includes apropshaft and a drive module. The drive module includes an input pinionshaft, a second differential, a pair of second axleshafts, which areadapted to be drivingly coupled to the pair of second vehicle wheels, atorque transfer device, and a limited slip clutch assembly. The seconddifferential has a second differential case and a pair of second outputgears that are driven by the second differential case. The second outputgears are drivingly coupled to the second axleshafts. The input pinionshaft is coupled by the propshaft to the output pinion shaft of thepower switching mechanism. The torque transfer device is operable in afirst switching mode, which inhibits transmission of rotary powerbetween the input pinion shaft and the second differential case, and ina second switching mode that permits transmission of rotary powerbetween the input pinion shaft and the second differential case. Thelimited slip clutch assembly is operable in a first clutch mode, whichpermits speed differentiation between the second differential case andone of the second axleshafts, and in a second clutch mode that inhibitsspeed differentiation between the second differential case and said oneof the second axleshafts.

In still another form, the present teachings provide a drivetrain for anall-wheel drive motor vehicle. The drivetrain includes a firstdriveline, a power switching mechanism and a second driveline. The firstdriveline is configured to drive a pair of first vehicle wheels andincludes a first differential and a pair of first axleshafts. The firstdifferential has a first differential case and a pair of first outputgears driven by the first differential case. The first axleshafts areconfigured to be drivingly coupled to the first output gears and to thefirst vehicle wheels. The power switching mechanism has an input shaftthat is configured to rotate with the first differential case, an outputpinion shaft, and a disconnect mechanism. The disconnect mechanism isoperable in a disconnected mode, which inhibits transmission of rotarypower between the input shaft and the output pinion shaft, and in aconnected mode that permits transmission of rotary power between theinput shaft and the output pinion shaft. The second driveline isconfigured to drive a pair of second vehicle wheels and includes apropshaft and a drive module. The drive module has an input pinion, acase, a ring gear, a torque transfer device, a pair of output members,and a limited slip clutch assembly. The propshaft couples the inputpinion shaft to the output pinion shaft of the power switchingmechanism. The case is rotatably disposed about a first axis that isperpendicular to a second rotational axis about which the input pinionshaft rotates. The ring gear is rotatable relative to the case. Thetorque transfer device is disposed about the first axis and isselectively operable for transmitting rotary power between the ring gearand the case. The output members are configured to transmit rotary powerin torque paths between the case and the second vehicle wheels. Thelimited slip clutch assembly is selectively operable in a mode thatrotationally couples one of the output members and the case.

Further areas of applicability will become apparent from the descriptionand claims herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and are not intended to limit the scope of thepresent disclosure in any way. Similar or identical elements are givenconsistent reference numerals throughout the various figures.

The present disclosure will become more fully understood from thedetailed description and the accompanying drawings wherein:

FIG. 1 is a schematic of a motor vehicle equipped with a disconnectableall-wheel drive system constructed in accordance with the presentteachings;

FIG. 2 is a schematic illustration of a power take-off unit associatedwith the disconnectable all-wheel drive system of FIG. 1;

FIG. 3 through FIG. 5 are perspective views of a power take-off unitbased on the schematic illustration shown in FIG. 2 with its housingstructure removed for improved clarity and which is constructed inaccordance with the present teachings;

FIG. 6 is an exploded perspective view of the power take-off unitconstructed in accordance with the present teachings;

FIGS. 7 and 8 are sectional views of the power take-off unit constructedin accordance with the present teachings;

FIG. 9 is a schematic illustration of a rear drive module associatedwith the disconnectable all-wheel drive system of FIG. 1;

FIGS. 10 through 11 are perspective views of a rear drive module basedon the schematic illustration shown in FIG. 9, with and without itshousing structure, and which is constructed in accordance with thepresent teachings;

FIG. 12 is a sectional view of the rear drive module constructed inaccordance with the present teachings;

FIG. 13 is an enlarged partial view of the rear drive module of FIG. 12showing the components associated with the torque transfer device ingreater detail; and

FIG. 14 is an enlarged partial view of the rear drive module of FIG. 12showing the components associated with the limited slip clutch assemblyin greater detail.

DETAILED DESCRIPTION

The following exemplary embodiments are provided so that the presentdisclosure will be thorough and fully convey the scope to those skilledin the art. Numerous specific details are set forth such as examples ofspecific components, devices and schematic configurations to provide athorough understanding of exemplary embodiments of the presentdisclosure. However, it will be apparent to those skilled in the artthat these specific details need not be employed, that the exemplaryembodiments may be embodied in many different forms, and that neithershould be construed to limit the scope of the present disclosure.

With reference to FIG. 1 of the drawings, a motor vehicle constructed inaccordance with the teachings of the present disclosure is schematicallyshown and generally indicated by reference numeral 10. The vehicle 10can include a powertrain 12 and a drivetrain 14 that can include aprimary driveline 16, a power switching mechanism 18, a secondarydriveline 20, and a control system 22. In the various aspects of thepresent teachings, the primary driveline 16 can be a front drivelinewhile the secondary driveline 20 can be a rear driveline.

The powertrain 12 can include a prime mover 24, such as an internalcombustion engine or an electric motor, and a transmission 26 which canbe any type of ratio-changing mechanism, such as a manual, automatic, orcontinuously variable transmission. The prime mover 24 is operable toprovide rotary power to the primary driveline 16 and the power transfermechanism 18.

With additional reference to FIG. 2, the primary driveline 16 caninclude a first or primary differential 30 having an input member 32driven by an output member (not shown) of the transmission 26. In theparticular construction shown, the first differential 30 is configuredas part of the transmission 26, a type commonly referred to as atransaxle and typically used in front-wheel drive vehicles. The primarydriveline 16 can further include a pair of first axleshafts 34L, 34Rthat can couple output components of the first differential 30 to afirst set of vehicle wheels 36L, 36R. The first differential 30 caninclude a first differential case 38 that is rotatably driven by theinput member 32, at least one pair of first pinion gears 40 rotatablydriven by the first differential case 38, and a pair of first outputgears 42 meshed with the first pinion gears 40 and which are connectedto drive the first axleshafts 34L, 34R.

With particular reference now to FIGS. 2 through 8, the power switchingmechanism 18, hereinafter referred to as a power take-off unit (PTU),can include a housing 46, an input 48 coupled for common rotation withthe first differential case 38 of the first differential 30, an output50, a transfer gear assembly 52, a disconnect mechanism 54, and anactuator 56. The input 48 can include a tubular input shaft 58 rotatablysupported by the housing 46 and which concentrically surrounds a portionof the first axleshaft 34R. A first end of the input shaft 58 can becoupled for rotation with the first differential case 38. The output 50can include an output pinion shaft 60 rotatably supported by the housing46 and having a pinion gear 62. The transfer gear assembly 52 caninclude a hollow gear shaft 64 and a hypoid gear 66 that is meshed withthe pinion gear 62. The gear shaft 64 can concentrically surround aportion of the input shaft 58 and can be rotatably supported by thehousing 46. The hypoid gear 66 can be integrally formed on, or fixed forcommon rotation with, the gear shaft 64 such as by bolts 68.

The disconnect mechanism 54 can comprise any type of clutch, disconnector coupling device that can be employed to selectively transmit rotarypower from the powertrain 14 to the secondary driveline 20. In theparticular example provided, the disconnect mechanism 54 is generallyconfigured as a dog clutch. The dog clutch can include a set of externalspline teeth 70 formed on a second end of the input shaft 58, a set offace clutch teeth 72 formed on the gear shaft 64, a mode collar 74having a set of internal spline teeth 76 constantly meshed with theexternal spline teeth 70 on the input shaft 58, and a shift fork 78operable to axially translate the mode collar 74 between a first modeposition and a second mode position. While schematically shown as asliding dog clutch, and shown more specifically in FIGS. 3 through 8 asa face-type dog clutch, it will be understood that the disconnectmechanism 54 can include any suitable dog clutch or selectivelyengageable coupling device if such an alternative configuration isdesired.

The mode collar 74 is shown in its first mode position, identified by a“2WD” leadline, wherein a set of face clutch teeth 80 formed on the modecollar 74 are disengaged from the face clutch teeth 72 on the gear shaft64. As such, the input shaft 58 is disconnected from driven engagementwith the gear shaft 64. Thus, no rotary power is transmitted from thepowertrain 12 through the transfer gear assembly 52 to the output pinionshaft 60 of the power take-off unit 18. With the mode collar 74 in itssecond mode position, identified by an “AWD” leadline, its face clutchteeth 80 are engaged with the face clutch teeth 72 on the gear shaft 64.Accordingly, the mode collar 74 establishes a drive connection betweenthe input shaft 58 and the gear shaft 64 such that rotary power from thepowertrain 12 is transmitted through the power take-off unit 18 to theoutput pinion shaft 60. As will be detailed, the output pinion shaft 60is coupled via a propshaft 86 to the secondary driveline 20.

The actuator 56 can be any type of actuator mechanism that is operablefor axially moving the shift fork 78 which, in turn, causes concurrentaxial translation of the mode collar 74 between its two distinct modepositions. The actuator 56 is shown mounted to the housing 46 of thepower take-off unit 18. The actuator 56 can be a power-operatedmechanism that can receive control signals from the control system 22and can include, for example, hydraulically-actuated,pneumatically-actuated or electromechanically-actuated arrangements.

As noted, FIG. 2 schematically illustrates the components that can beassociated with the power take-off unit 18. Reference now to FIG. 3through 8 will provide a more definitive structural configuration ofsuch components that are associated with an exemplary embodiment of thepower take-off unit 18. In particular, some of these figures illustratethe components in an assembled condition with portions of the housing 46removed for improved clarity. Each of the input shaft 58, the gear shaft64, and the output pinion shaft 60 are shown with suitable bearingsassembled thereon for rotatably supporting each within or from thehousing 46. The actuator 56 is shown as a self-contained power-operatedunit 82 from which an axially moveable plunger 84 extends and to which acylindrical hub portion 86 of the mode fork 74 is secured. Thepower-operated unit 82 can include an electromagnetic drive unit, suchas a solenoid, configured to extend and retract the plunger 84 forcausing concurrent translational movement of the shift fork 74. A returnspring 88 is configured to assist in retracting the plunger 84 in apower-off (fail safe) condition of the power unit 82. External splineteeth 90 are formed on one end of the tubular input shaft 58 tofacilitate a splined connection with a splined portion (not shown) ofthe first differential case 38. It can also be seen that the face clutchteeth 72 are formed on an enlarged annular boss portion 92 of the gearshaft 64 to provide for increased rigidity.

With particular reference now to FIGS. 1 and 9, the secondary driveline20 can include the propshaft 86, a rear axle drive module (RDM) 100, apair of second axleshafts 102L, 102R, and a set of secondary vehiclewheels 104L, 104R. A first end of the propshaft 86 can be coupled forrotation with the output pinion shaft 60 extending from the powertake-off unit 18 while a second end of the propshaft 86 can be coupledfor rotation with an input 106 of the rear drive module 100. The reardrive module 100 can generally include a housing 108, a second orsecondary differential 110, a torque transfer device (TTD) 112, a TTDactuator 114, a limited slip clutch (LSC) assembly 116, and a LSCactuator 118.

The input 106 can include an input pinion shaft 120 having a pinion gear122, a ring gear housing 124, and a ring gear 126 fixed for rotationwith the ring gear housing 124 and which is meshed with the pinion gear122. The second differential 110 can include a second differential case130, at least one pair of second pinion gears 132 rotatably driven bythe second differential case 30, and a pair of second output gears 134that are meshed with the second pinion gears 132. The second outputgears 134 are fixed for rotation with the inboard ends of the secondaxleshafts 102L, 102R.

The torque transfer device 112 can include any type of clutch orcoupling device that can be employed to selectively transmit rotarypower from the input 106 to the second differential case 130 of thesecond differential 110. In the example shown, the torque transferdevice 112 is a multi-plate friction clutch that can include an inputclutch member 140 driven by the ring gear housing 124, an output clutchmember 142 coupled for rotation with the second differential case 130, amulti-plate clutch pack 144 having interleaved friction plates disposedbetween the input and output clutch members, and an engagement member146 that is moveable for selectively applying a clutch engagement forceto the clutch pack 144. The TTD actuator 114 is configured to generatetranslational movement of the engagement member 146 relative to theclutch pack 144 and can be controlled in response to control signalsfrom the control system 22.

A first or “disconnected” mode can be established for the torquetransfer device 112 when the engagement member 146 is positioned suchthat rotary power is not transmitted from the input clutch member 140 tothe output clutch member 142. In this “disconnected” mode, the secondaryvehicle wheels 104L, 104R, the second axleshafts 102L, 102R and thesecond differential 110 are disconnected from the input 106 of the reardrive module 100. As such, rotation of these components resulting fromrolling motion of the secondary vehicle wheels 104L, 104R does not“back-drive” the input 106 of the rear drive module 100, the propshaft86, and the output components of the power take-off unit 18.

A second or “connected” mode for the torque transfer device 112 can beestablished when the clutch engagement force exerted by the engagementmember 146 on the clutch pack 144 causes rotary power to be transmittedfrom the input 106 to the second differential case 130 for delivery tothe secondary vehicle wheels 104L, 104R through the second differential110. In addition, a “torque biasing” function can also be provided inthe connected mode since variable control over the magnitude of theclutch engagement force applied to the clutch pack 144 can vary thedistribution ratio of the rotary power transmitted from the powertrain12 to the primary driveline 16 and the secondary driveline 20. Thus, thetorque transfer device 112 can be configured or controlled to slip orcyclically engage and disengage as appropriate for biasing the availabledrive torque while establishing the drive connection between the input106 and the second differential 110.

The TTD actuator 114 can be any power-operated device capable ofshifting the torque transfer device 112 between its first and secondmodes as well as adaptively regulating the magnitude of the clutchengagement force exerted by the engagement member 146 on the clutch pack144. Thus, the TTD actuator 114 can, for example, include anelectromagnetic or motor-driven ballscrew, ballramp or other camactuation system having a mechanical connection, shown by lead line 150,with the engagement member 146. Alternatively, the TTD actuator 114 caninclude a hydraulic actuation system capable of regulating the positionof the engagement member 146 relative to the clutch pack 144 byregulating fluid pressure, also indicated by the lead line 150,delivered to a pressure chamber.

The limited slip clutch assembly 116 can include any type of clutch orcoupling device that can be employed to selectively limit speeddifferentiation between the second differential case 130 and the secondaxleshaft 102R. In the example shown, the limited slip clutch assembly116 is a multi-plate friction clutch that can include an input clutchcomponent 152 driven by the second differential case 130, an outputclutch component 154 coupled for rotation with the second axleshaft102R, a multi-plate clutch pack 156 having interleaved friction platesdisposed between the input and output clutch components, and anactuation mechanism 158 that is moveable for selectively applying aclutch engagement force to the clutch pack 156. The LSC actuator 118 isprovided to generate translational movement of a component of theactuation mechanism 158 relative to the clutch pack 156 and can becontrolled by control signals from the control system 22.

A first or open differential mode can be established when the actuationmechanism 158 is positioned such that the second axleshaft 102R ispermitted to rotate relative to the second differential case 130 withoutfrictional resistance transmitted through the clutch pack 156. In thisopen differential mode, the rotary power transferred by the torquetransfer device 112 to the secondary differential 110 is transmitted tothe second vehicle wheels 104L, 104R based on the tractive roadconditions.

A second or locked differential mode can also be established when theclutch engagement force exerted by the actuation mechanism 158 on theclutch pack 156 is of sufficient magnitude to prevent relative rotationbetween the second axleshaft 102R and the second differential case 130.With the second differential 110 locked, the axleshafts 102L and 102Rare prevented from relative rotation and the rotary power transmittedthrough the torque transfer device 112 is divided equally to thesecondary vehicle wheels 104L, 104R. In addition, a “side-to-side”torque biasing function can also be provided in the locked differentialmode since variable control over the magnitude of the clutch engagementforce applied to the clutch pack 156 can vary the distribution ratio ofthe rotary power transmitted through the second differential 110 to eachof the secondary wheels 104L, 104R. Accordingly, the limited slip clutchassembly 116 can be configured or controlled to slip or cyclicallyengage and disengage as appropriate for biasing the side-to-side torquetransfer between the secondary vehicle wheels 104.

The LSC actuator 118 can be any power-operated device capable ofshifting the limited slip clutch assembly 116 between its first andsecond modes as well as adaptively regulating the clutch engagementforce exerted on the clutch pack 156. The LSC actuator 118 can, forexample, include an electromagnetically-actuated or motor-drivenballscrew, ballramp or other cam actuated system having a mechanicalconnection, shown by lead line 160. Alternatively, the LSC actuator 118can include a hydraulic actuation system capable of regulating thehydraulic pressure exerted by the actuation mechanism 158 on the clutchpack 156. While shown as separate devices, it is also contemplated thata common actuator arrangement can be used to coordinate actuation of thetorque transfer device 112 and the limited slip clutch assembly 116.

The control system 22 is schematically shown in FIG. 1 to include acontroller 170, a group of first sensors 172, and a group of secondsensors 174. The group of first sensor 172 can be arranged within themotor vehicle 10 to sense a vehicle parameter and responsively generatea first sensor signal. The vehicle parameter can be associated with anycombination of the following: vehicle speed, yaw rate, steering angle,engine torque, wheel speeds, shaft speeds, lateral acceleration,longitudinal acceleration, throttle position and gear position withoutlimitations thereto. The group of second sensors 174 can be configuredto sense a driver-initiated input to one or more on-board devices and/orsystems within the vehicle 10 and responsively generate a second sensorsignal. For example, the motor vehicle 10 may be equipped with a modesensor associated with a mode selection device, such as a push button ora lever, that senses when the vehicle operator has selected betweenvehicle operation in a two-wheel drive (FWD) mode, an all-wheel drive(AWD) mode, and an all-wheel drive-locked (AWD-LOCK) mode. Also,switched actuation of vehicular systems such as the windshield wipers,the defroster, and/or the heating system, for example, may be used bythe controller 170 to assess whether the motor vehicle 10 should beshifted automatically between the FWD and AWD modes.

As noted, FIG. 9 schematically illustrates the components that can beassociated with the rear drive module 100. Referring now to FIGS. 10through 14, a more definitive structural configuration of suchcomponents associated with an exemplary embodiment of the rear drivemodule 100 is shown. The housing 108 can have at least three sectionsincluding a main housing section 180, a TTD housing section 182, and aLSC housing section 184 secured together via suitable bolts. The input106 and the second differential 110 are installed and rotatablysupported within an internal cavity formed in the main housing section180. The ring gear housing 124 has the ring gear 126 bolted thereto andcan include a set of internal splines 186 that are mated with a set ofexternal splines 188 formed on a clutch drum 190 which defines the inputclutch member 140 of the torque transfer device 112. The clutch drum 190is a two-piece assembly including a radial drum plate 189 and acylindrical drum 191 to which the outer friction clutch plates arecoupled.

The second differential case 130 of the second differential 110 isrotatably supported by a pair of laterally-spaced bearing 192 within theouter differential housing 124 and can include a first tubular boss 194that extends into the torque transfer device 112 and a second tubularboss 196 that extends into the limited slip clutch assembly 116. Aclutch hub 198 can be coupled (i.e., splined) for rotation with thefirst boss 194 and defines the output clutch member 142 of the torquetransfer device 112. The engagement member 146 can be a hydraulic pistonassembly 200 that is slidably disposed with a pressure chamber 202formed in the TTD housing section 182 for movement relative to theclutch pack 144. The TTD actuator 114 can include a pump assembly 204operated via the control system 22 to generate and regulate thehydraulic fluid pressure delivered from an accumulator 206 to thepressure chamber 202. A pressure transducer 208, associated with thefirst sensors 172, can be provided to detect the fluid pressure in thepressure chamber 202 and to transmit a sensor signal to the controller170.

The limited slip clutch assembly 116 is best shown in FIGS. 12 and 14and can be configured as an electromagnetically-actuated ballrampclutch. The input clutch component 152 can include a clutch drum 210having a set of internal splines 212 that are mated with a set ofexternal splines 214 formed on the second boss 196 of the seconddifferential case 130. The output clutch component 154 can include aclutch hub 216 having a set of internal splines 218 that are mated witha set of external splines 220 formed on the second axleshaft 102R. Theactuation mechanism 158 can include a ballramp unit 222 and a pilotclutch 224 that is disposed between the ballramp unit 222 and the clutchdrum 210. The LSC actuator 118 can include an electromagnetic coil 226and an armature plate 228 between which the pilot clutch 224 is located.Control signals sent from the controller 170 to the electromagnetic coil226 can function to cause the armature plate 228 to translate and engagethe pilot clutch 224 which, in turn, activates the ballramp unit 222 forcausing axial movement of an apply plate 230 relative to the clutch pack156. As such, control over the axial position of the apply plate 230controls the magnitude of the clutch engagement force exerted on theclutch pack 156 for establishing the first and second modes of thelimited slip clutch assembly 116. When operating in its second mode, theclutch engagement force applied by the apply plate 230 on the clutchpack 156 functions to limit relative rotation between the seconddifferential case 130 (via the clutch drum 210) and the second axleshaft102R (via the clutch hub 216).

With reference to FIGS. 1, 2 and 9, the vehicle 10 can normally beoperated in the two-wheel drive (FWD) mode in which the power take-offunit 18 and the rear drive module 100 are both disengaged. Specifically,the mode collar 74 of the disconnect mechanism 54 is positioned by theactuator 56 in its first mode position such that the input shaft 58 isuncoupled from the gear shaft 64. As such, substantially all powerprovided by the powertrain 12 is transmitted to the primary driveline16. Likewise, the torque transfer device 112 can be shifted into andmaintained in its first (disconnected) mode such that the input 106, thepropshaft 86, the output pinion shaft 60 and the transfer gear assembly52 within the power take-off unit 18 are not back-driven due to rollingmovement of the secondary wheels 104. The limited slip clutch assembly116 can also be maintained in its first (open differential) mode whenthe vehicle is operating in the two-wheel drive mode. There may besituations during operation of the vehicle 10 in the two-wheel drivemode when it would be beneficial, for vehicle dynamics purposes (e.g.yaw dampening), to actuate the limited slip clutch assembly 116 evenwhen no drive torque is transmitted to the secondary driveline 20. Thus,the controller 170 can control actuation of the LSC actuator 118 toshift the limited slip clutch assembly 116 into its second mode andadaptively regulate speed differentiation between the second vehiclewheels 104L, 104R.

When it is desired or necessary to operate the motor vehicle 10 in theall-wheel drive (AWD) mode, the control system 22 can be activated via asuitable input which, as noted, can include a drive requested input (viathe mode select device) and/or an input generated by the controller 170in response to signals from the first sensors 172 and/or the secondsensors 174. The controller 170 initially signals the TTD actuator 114to shift the torque transfer device 112 into its second (connected)mode. Specifically, the controller 170 controls operation of the TTDactuator 114 such that the actuation member 146 is moved and a clutchengagement force is exerted on the clutch pack 144 that is sufficient tosynchronize the speed of the secondary driveline 20 with the speed ofthe primary driveline 16. A speed sensor 240 (FIGS. 10 and 11) candetect the rotary speed of the input pinion shaft 116 and send theindicated speed signal to the controller 170 for use in determiningspeed synchronization. Upon speed synchronization, the controller 170signals the actuator 56 to cause the mode collar 74 in the powertake-off unit 18 to move from its first mode position into its secondmode position. With the mode collar 74 in its second mode position,rotary power is transmitted from the powertrain 12 to the primarydriveline 16 and the secondary driveline 20. It will be appreciated thatsubsequent control of the magnitude of the clutch engagement forcegenerated by the torque transfer device 112 permits torque biasingacross the clutch pack 144 for controlling the torque distribution ratiotransmitted from the powertrain 12 to the primary driveline 16 and thesecondary driveline 20.

When it is desired or necessary to operate the vehicle 10 in itsall-wheel drive-locked (AWD-LOCK) mode, the control system 22 can signalthe LSC actuator 118 to shift the limited slip clutch assembly 116 fromnormal operation in its first mode into its second mode. As noted, suchaction causes the actuation mechanism 158 to engage the clutch pack 156and, depending on the magnitude of the clutch engagement force, limit ortotally inhibit speed differentiation between the second axleshafts102L, 102R. It is contemplated that the mode selector could permit thevehicle operator to select the AWD-LOCK mode when the vehicle 10 isoperating off-road or is struck in mud or snow. As an alternative,actuation of the limited slip clutch assembly 116 can be totallyautomatic without input from the vehicle operator. In either scenario,the limited slip clutch assembly 116 provides enhanced off-road tractionperformance and driving dynamic capability in addition to a yaw dampingfeature.

While specific aspects have been described in the specification andillustrated in the drawings, it will be understood by those skilled inthe art that various changes can be made and equivalents can besubstituted for elements and components thereof without departing fromthe scope of the present teachings, as defined in the claims.Furthermore, the mixing and matching of features, elements, componentsand/or functions between various aspects of the present teachings areexpressly contemplated herein so that one skilled in the art willappreciate from the present teachings that features, elements,components and/or functions of one aspect of the present teachings canbe incorporated into another aspect, as appropriate, unless describedotherwise above. Moreover, many modifications may be made to adapt aparticular situation, configuration, or material to the presentteachings without departing from the essential scope thereof. Therefore,it is intended that the present teachings not be limited to theparticular aspects illustrated by the drawings and described in thespecification as the best mode presently contemplated for carrying outthe present teachings, but that the scope of the present teachingsinclude many aspects and examples following within the foregoingdescription and the appended claims.

What is claimed is:
 1. A drivetrain for an all-wheel drive motorvehicle, the drivetrain comprising: a first driveline that is adapted todrive a pair of first vehicle wheels, the first driveline including afirst differential and a pair of first axleshafts, the firstdifferential having a first differential case and a pair of first outputgears driven by the first differential case, the first axleshafts beingdrivingly coupled to the first output gears and to the first vehiclewheels; a power switching mechanism having an input shaft, an outputpinion shaft, and a disconnect mechanism, the input shaft being drivenby the first driveline, the disconnect mechanism being operable in adisconnected mode, which inhibits transmission of rotary power betweenthe input shaft and the output pinion shaft, and in a connected modethat permits transmission of rotary power between the input shaft andthe output pinion shaft; and a second driveline that is adapted to drivea pair of second vehicle wheels, the second driveline including apropshaft and a drive module, the drive module including an input pinionshaft, a second differential, a pair of second axleshafts, which areadapted to be drivingly coupled to the pair of second vehicle wheels, atorque transfer device, and a limited slip clutch assembly, the seconddifferential having a second differential case and a pair of secondoutput gears that are driven by the second differential case, the secondoutput gears being drivingly coupled to the second axleshafts, the inputpinion shaft being coupled by the propshaft to the output pinion shaftof the power switching mechanism, the torque transfer device beingoperable in a first switching mode, which inhibits transmission ofrotary power between the input pinion shaft and the second differentialcase, and in a second switching mode that permits transmission of rotarypower between the input pinion shaft and the second differential case,the limited slip clutch assembly being operable in a first clutch mode,which permits speed differentiation between the second differential caseand one of the second axleshafts, and in a second clutch mode thatinhibits speed differentiation between the second differential case andsaid one of the second axleshafts.
 2. The drivetrain of claim 1 whereinthe power switching mechanism is a power take-off unit having a transfergear assembly that drives the output pinion shaft, and wherein thedisconnect mechanism comprises a clutch that is selectively operable torotationally couple an input gear of the transfer gear assembly to theinput shaft.
 3. The drivetrain of claim 2, wherein the clutch comprisesa mode collar that is coupled for common rotation with the input shaft,the mode collar being slidable on the input shaft for movement between afirst mode position and a second mode position, wherein the mode collaris decoupled from the input gear when the mode collar is positioned inits first mode position, and wherein the mode collar connects the inputgear to the input shaft when the mode collar is positioned in its secondmode position.
 4. The drivetrain of claim 3 wherein the input gearcomprises a gear shaft, which is rotatably disposed about the inputshaft, and a ring gear fixed to the gear shaft, the ring gear beingmeshed with a pinion gear fixed to the output pinion shaft.
 5. Thedrivetrain of claim 4, wherein a set of first face clutch teeth areformed on the gear shaft and a set of second face clutch teeth areformed on the mode collar, and wherein the second face clutch teeth arein meshed engagement with the first face clutch teeth when the modecollar is positioned in its second mode position.
 6. The drivetrain ofclaim 1, wherein the input pinion shaft drives a ring gear for rotationabout a rotational axis and wherein the torque transfer device comprisesa first friction clutch with a first set of clutch plates that arerotatably disposed about the rotational axis.
 7. The drivetrain of claim6, wherein the limited slip clutch assembly comprises a second frictionclutch with a second set of clutch plates that are rotatably disposedabout the rotational axis.
 8. The drivetrain of claim 7, wherein thefirst and second friction clutches are disposed on opposite sides of thesecond differential.
 9. The drivetrain of claim 7, wherein the limitedslip clutch assembly further comprises an actuator for selectivelyoperating the second friction clutch.
 10. The drivetrain of claim 9,wherein the actuator comprises a ball-ramp actuator.
 11. The drivetrainof claim 10, wherein the actuator further comprises an electromagnetthat is selectively operable for controlling operation of the ball-rampactuator.
 12. A drivetrain for an all-wheel drive motor vehicle, thedrivetrain comprising: a first driveline that is adapted to drive a pairof first vehicle wheels, the first driveline including a firstdifferential and a pair of first axleshafts, the first differentialhaving a first differential case and a pair of first output gears drivenby the first differential case, the first axleshafts being drivinglycoupled to the first output gears and to the first vehicle wheels; apower switching mechanism having an input shaft, and an output pinionshaft, and a disconnect mechanism, the input shaft being driven by thefirst driveline, the disconnect mechanism being operable in adisconnected mode, which inhibits transmission of rotary power betweenthe input shaft and the output pinion shaft, and in a connected modethat permits transmission of rotary power between the input shaft andthe output pinion shaft; and a second driveline that is adapted to drivea pair of second vehicle wheels, the second driveline including apropshaft and a drive module, the drive module including an input pinionshaft, a second differential, a pair of second axleshafts, which areadapted to be drivingly coupled to the pair of second vehicle wheels, atorque transfer device, and a limited slip clutch assembly, the seconddifferential having a second differential case and a pair of secondoutput gears that are driven by the second differential case, the secondoutput gears being drivingly coupled to the second axleshafts, the inputpinion shaft being coupled by the propshaft to the output pinion shaftof the power switching mechanism, the torque transfer device beingoperable in a first switching mode, which inhibits transmission ofrotary power between the input pinion shaft and the second output gears,and in a second switching mode that permits transmission of rotary powerbetween the input pinion shaft and the second output gears, the limitedslip clutch assembly being operable in a first clutch mode, whichpermits speed differentiation between the second differential case andone of the second axleshafts, and in a second clutch mode that inhibitsspeed differentiation between the second differential case and said oneof the second axleshafts.
 13. The drivetrain of claim 12 wherein thepower switching mechanism is a power take-off unit having a transfergear assembly that drives the output pinion shaft, and wherein thedisconnect mechanism comprises a clutch that is selectively operable torotationally couple an input gear of the transfer gear assembly to theinput shaft.
 14. The drivetrain of claim 13, wherein the clutchcomprises a mode collar that is coupled for common rotation with theinput shaft, the mode collar being slidable on the input shaft formovement between a first mode position and a second mode position,wherein the mode collar is decoupled from the input gear when the modecollar is positioned in its first mode position, and wherein the modecollar connects the input gear to the input shaft when the mode collaris positioned in its second mode position.
 15. The drivetrain of claim14 wherein the input gear comprises a gear shaft, which is rotatablydisposed about the input shaft, and a ring gear fixed to the gear shaft,the ring gear being meshed with a pinion gear fixed to the output pinionshaft.
 16. The drivetrain of claim 15 wherein a set of first face clutchteeth are formed on the gear shaft and a set of second face clutch teethare formed on the mode collar, and wherein the second face clutch teethare in meshed engagement with the first face clutch teeth when the modecollar is positioned in its second mode position.
 17. The drivetrain ofclaim 12 wherein the input pinion shaft drives a ring gear for rotationabout a rotational axis and wherein the torque transfer device comprisesa first friction clutch with a first set of clutch plates that arerotatably disposed about the rotational axis.
 18. The drivetrain ofclaim 17 wherein the limited slip clutch assembly comprises a secondfriction clutch with a second set of clutch plates that are rotatablydisposed about the rotational axis.
 19. The drivetrain of claim 18wherein the first and second friction clutches are disposed on oppositesides of the second differential.
 20. The drivetrain of claim 18 whereinthe limited slip clutch assembly further comprises an actuator forselectively operating the second friction clutch.
 21. The drivetrain ofclaim 20 wherein the actuator comprises a ball-ramp actuator.
 22. Thedrivetrain of claim 21 wherein the actuator further comprises anelectromagnet that is selectively operable for controlling operation ofthe ball-ramp actuator.
 23. A drivetrain for an all-wheel drive motorvehicle, the drivetrain comprising: a first driveline that is adapted todrive a pair of first vehicle wheels, the first driveline including afirst differential and a pair of first axleshafts, the firstdifferential having a first differential case and a pair of first outputgears driven by the first differential case, the first axleshafts beingdrivingly coupled to the first output gears and to the first vehiclewheels; a power switching mechanism having an input shaft, and an outputpinion shaft, the input shaft being driven by the first driveline; and asecond driveline that is adapted to drive a pair of second vehiclewheels, the second driveline including a propshaft and a drive module,the drive module including an input pinion shaft, a second differential,a pair of second axleshafts, which are adapted to be drivingly coupledto the pair of second vehicle wheels, a torque transfer device, and alimited slip clutch assembly, the second differential having a seconddifferential case and a pair of second output gears that are driven bythe second differential case, the second output gears being drivinglycoupled to the second axleshafts, the input pinion shaft being coupledby the propshaft to the output pinion shaft of the power switchingmechanism, the torque transfer device being operable in a firstswitching mode, which inhibits transmission of rotary power between theinput pinion shaft and the second output gears, and in a secondswitching mode that permits transmission of rotary power between theinput pinion shaft and the second output gears, the limited slip clutchassembly being operable in a first clutch mode, which permits speeddifferentiation between the second differential case and one of thesecond axleshafts, and in a second clutch mode that inhibits speeddifferentiation between the second differential case and said one of thesecond axleshafts.